This invention relates to detection of fluorescent radiation from articles including documents marked with materials that fluoresce when illuminated with non-visible radiation such as ultraviolet radiation. The invention is particularly applicable to detection of counterfeit documents or other counterfeit products and detection of documents or other products which have been subjected to tampering, forging, or other unauthorized modification. Thus the invention is particularly useful for confirming the authenticity and integrity of articles, including valuable documents such as legal papers, identification cards, credit cards, licenses, passports, customs and immigration documents, and valuable mail. The invention can also be applied to quality control of products, providing quantitative and statistical information where the presence or quantity of particular materials is important. Some other examples of applications include label verification, safety seal verification, article alignment, and sorting of articles into two or more categories.
Many methods have been known in the prior art to authenticate valuable articles. Some known methods include imprinting on the articles a white-light hologram or imprinting reflective and diffractive indicia displaying distinctive images that are difficult to counterfeit. Other known methods include incorporation of distinctive fibers into the articles, such fibers being detectable by visual observation, microwave irradiation, or other means. U.S. Pat. No. 4,92 1,280 describes fibers made luminescent by a dyeing process employing rare-earth compounds, which fibers may be incorporated into articles requiring authentication.
It is well-known in the prior art that documents may be authenticated by marking the documents with substances such as inks or dyes that appear invisible or relatively unnoticeable to the naked eye in ordinary visible illumination, but that fluoresce when illuminated with ultraviolet light, revealing marks that serve to identify the legitimate document. These methods depend on substances that are not easily or inexpensively identified by a counterfeiter, and not easily or inexpensively duplicated or mimicked by a counterfeiter. When using these methods, it is desirable to use substances such as dyes or inks that fluoresce in narrow spectral bands which are distinguishable by sufficiently narrow-band detectors, but not readily distinguishable by eye. In U.S. Pat. No. 4,146,792 by Stenzel et al., these methods are extended to include dyes containing rare-earth elements whose fluorescence is influenced by the chemical environment of the fluorescing atoms in a non-fluorescing matrix, and the detection is refined to include detection of predetermined fine structure in the line spectrum of emitted light. In this method, the spectral free structure is built into the marking dyes by the formulation of the dyes, and the corresponding discrimination of fine spectral structure is fixed in the physical structure of the checking device by the sizes and positions of photocells cooperating with an optical dispersion element, particularly a narrow-bandwidth interference filter. Yet another class of authentication methods uses substances which fluoresce in the infrared portion of the electromagnetic spectrum when illuminated by light in the visible portion of the spectrum.
In U.S. Pat. No. 4,642,526 by Hopkins and assigned to the assignee of the present invention, a source of ultraviolet light is made self-modulating at a predetermined frequency. Detection of the secondary radiation, filtering of the detected signal, and demodulation of the filtered signal at the predetermined frequency allow the system of Hopkins' invention to detect the fluorescent marks despite interference from ambient light sources.
Marking products with indicia such as bar codes using fluorescent substances such as inks or dyes is also known in the prior art, both for the purposes described above and for providing identification on the products without detracting from the products' appearance as normally viewed in visible light. U.S. Pat. No. 4,983,817 describes methods and apparatus for reading bar codes printed with fluorescent substances, while compensating for variations in background reflectance over the area printed with the bar code.
In many of the known authentication methods using fluorescence, the fluorescent identifying substance may be incorporated into the article during the article's manufacture instead of marking the article afterwards. One example is incorporating fluorescent substances such as dyes into paper during its manufacture and then using such paper for valuable documents.
While the various known methods of authenticating articles are useful for many purposes, there is a need for an authentication system that is more readily adaptable by the user, that is reliable, and that provides fast authentication in the presence of optical and electrical noise and bright ambient lighting.